Pathogenic fungi colonizing human skin, so-called dermatophytes, are a pleague for millions of humans. There are highly specialized filamentous fungi exclusively infecting keratinized animal structures causing mycoses. Studies of dermatophyte pathogenic interactions have been hampered by a lack of full genome sequences. To provide broad insights into the molecular basis of the pathogenicity-associated traits, Burmester et al. published in the last issue of Genome Biology (2011/12/1/R7) the first genome sequences of two closely phylogenetically related dermatophytes, Arthroderma benhamiae and Trichophyton verrucosum, both of which induce highly inflammatory infections in humans.
The genomes of A. benhamiae and T. verrucosum were sequenced by a whole-genome shotgun hybrid approach (Sanger, 454 FLX). These genomes are smaller than those of phylogenetically related ascomycetes. The assembly of A. benhamiae spans 22.3 Mb and that of T. verrucosum comprises 22.6 Mb. The genomes of A. benhamiae and T. verrucosum are compact and only contain 7,980 and 8,024 predicted protein-encoding genes, respectively. Most of these genes lie in collinear regions and are shared between the two fungi suggesting a very recent speciation.
As expected from their peculiar ecological niche, i.e. the animal skin, the two dermatophytes belong to the most protease-rich fungal species. They contain 235 protease-encoding genes, 87 of the predicted proteins having a secretion signal. A comprehensive analysis of the secretome during keratin degradation was carried out by combining 2D-PAGE and MALDI-TOF/TOF. RNA-Seq transcriptome profiling of A. benhamiae growing on human keratinocytes was performed to investigate the the entire process of infection. Subtilisin-like serine proteases, fungalysine-type metalloproteases, leucine aminopeptidases and dipeptidyl-peptidases are secreted during growth of A. benhamiae on keratin and keratinocytes. In addition, these dermatophytes can efficiently assimilate lipids, major constituents of the skin, thanks to the presence of 16 lipase genes. As discussed by the authors, the presence of large protease gene families in dermatophytes and their striking induction on keratin and human keratinocytes likely reflects selection during evolution and the ability of these fungi to adapt to different environmental conditions during infection and saprophytic growth. It appears that secondary metabolites also play a crucial role during keratinocyte infection.
Burmester et al. (2011) Comparative and functional genomics provide insights into the pathogenicity of dermatophytic fungi. Genome Biology 12:R7
Photo: A. benhamiae on human hair © Burmester et al.